1. Field
The present disclosure relates to an improved threaded fastener and method for securing workpieces of composite materials. In particular embodiments, the present disclosure relates to a fastener including a pin and a preformed sleeve which may be placed around the pin for use in interference applications.
2. General Background
More and more graphite composite materials are being incorporated into aircraft structures. Use of graphite composites increase strength, increase life, reduce weight, reduce fuel consumption, increase payload, among other benefits. However, as these newer materials are utilized, new challenges need to be overcome in fastening technology when compared to typical metallic structures.
Existing aerospace fasteners cannot be safely installed in interference conditions in graphite or mixed graphite-composite metallic structures. Typically, clearance fit fasteners are utilized to avoid concerns of composite delamination and potential structural failure that make these fasteners unsafe to utilize. As a result, fasteners are installed in clearance holes that result in the reduction of dynamic joint performance, gaps in the structure, and other structural concerns.
The resulting gap between the fastener shank portion and hole prevent uniform contact of structural components. Consequently, safe dissipation of lightning strike current/energy, and electro-magnetic currents is a major concern. Currently, aircraft manufacturers are resorting to elaborate, expensive, and sometimes risky alternative methods to properly ground the structure. For example, copper, or another low conductive strip, may be incorporated onto the surface of the workpieces to provide a preferential low resistance path for any current. Additionally, a film adhesive containing a conductive fiber carrier film capable of conducting high currents between two workpieces may be utilized. However, both of these methods are very expensive and not a cost effective way to provide safe dissipation of current.
Additionally, prior fasteners cannot be installed with significant amounts of sealant, as is required in most aircraft structures. If sufficient sealant is utilized during installation, the coefficient of friction between the fastener assembly and the workpieces is reduced hindering installation capability. Additionally, there is an inability to flow any excess sealant out of the joint.
Further, previous pins with mating sleeves manufactured for interference applications are only capable of being installed in 100% graphite composite materials. In addition, these fasteners are limited to applications of short lengths and small diameters. Prior fasteners cannot be installed in any composite/metallic structures and most 100% percent composite structures.
Additionally, previous fasteners for interference applications are only available in shear load range strength capabilities. The collars utilized in these fasteners are typically commercially pure titanium and subject to creep at fairly low elevated temperatures.
Thus, there is a need to provide a fastener that allows for interference applications without the possibility of delamination and structural failure.
There is also a need to provide a fastener that may be used in a variety of different applications. Utilization of a fastener in a variety of composite/metallic structures is needed.
Additionally, there is a need to provide fasteners which provide safe dissipation of electrical currents caused by lightning strikes and/or static electricity. Fasteners that allow for uniform contact of the structural components will provide the necessary dissipation and a safer, more cost effective solution to the problems involving electrical currents.